1. Field of the Invention
This invention relates to a vehicle rear wheels steering apparatus.
2. Related Background Art
In recent years, a rear wheels steering apparatus in a vehicle, especially an automobile, has been developed, and some have been already put into the market. In Japanese Patent Laid-Open Publication No. 61-163064, an example of such a rear wheels steering apparatus has been described. The rear wheels steering apparatus will be described referring to FIG. 7.
FIG. 7 is a sectional view of a rear wheels steering apparatus according to the prior art. A worm 702 is coaxially mounted to an input shaft 701 whose front end is connected with the front wheels side steering mechanism (not shown). The worm 702 engages a worm wheel 703. The worm wheel 703 is connected with a double cylinder 704 by intersecting the mutual axes at right angles with each other, and when the worm wheel 703 rotates, the double cylinder is constructed to pivotally operate clockwise or couterclockwise.
The double cylinder 704 consists of an outer cylinder 704a and an inner cylinder 704b, and the inner cylinder 704b may freely rotate relative to the outer cylinder 704a. An opening 704c is provided at a part of the inner cylinder 704b, and a shaft 705 pivotally engages this opening 704c. The shaft 705 is installed to the outer periphery of a rod 706, and moves in the axial direction of the rod together with the rod 706. Both ends of the rod 706 are connected with a steering mechanism (not shown), and the wheels (not shown) are constructed so that they are steered in the axial direction of the rod 706. It is assumed here that when the rod 706 moves to the left, the rear wheels are steered to the left, and when the rod moves to the right, the rear wheels are steered to the right.
The rod 706 is rotatably supported around the axis, and the rotating force of a motor 708 is transmitted to the rod 706 through a worm mechanism 707. When the rod 706 rotates, the shaft 705 also rotates around the axis of the rod 706 accordingly.
The operation of the rear wheels steering apparatus according to the prior art will be described below.
At a position of the shaft 705 (turned upward) shown in FIG. 7, the input shaft 701 connected with the front wheels side steering mechanism is rotated by the driver's steering operation to rotate the worm wheel 703 counterclockwise. Thereby the shaft 705 moves to the left in FIG. 7 together with the rod 706, and the rear wheels are steered to the left, that is, in the normal phase to the front wheels and by an amount in proportion to the amount of steering of the front wheels.
On the other hand, assuming that the rod 706 is rotated by the rotation of the motor 708 to move the shaft 705 to a position opposite (turned downward) to the position shown in FIG. 7, when the worm wheel 703 lockwise by the driver's steering operation likewise, the shaft 705 moves to the right together with the rod 706, and the rear wheels are steered to the right, that is, in the reverse phase to the front wheels and by an amount in proportion to the amount of steering of the front wheels.
As can be seen from the above description, when the shaft 705 is perpendicular to the plane of FIG. 7, the rod 706 does not move according to the rotation of the worm wheel 703, and the steering of the rear wheels is not performed, irrespective of the driver's steering operation. That is, the amount of steering of the rear wheels can be continuously changed from the maximum amount in the normal phase in proportion to an amount of steering of the front wheels to zero and from zero to the maximum amount in the opposite phase by controlling the position of the shaft 705 by means of the motor 708.
In such a conventional rear wheels steering apparatus, however, a part of the steering torque to steer the front wheels is mechanically transmitted to rotate the worm wheel 703. Therefore, mechanical transmitting means for the steering torque similar to a propeller shaft will be required. Since, however, space is greatly restricted in a passenger car, the use of the mechanical transmitting means will narrow the inside of the car room, for which a large volume is required, increase its weight, and further cause a considerable amount of loss in torque due to the addition of a sliding portion.
With reference to such a conventional technique, a construction, in which a main motor and a sub-motor are used to rotate the rod and the double cylinder for control, is considered. In this construction, however, a reduction gear connected with the main motor or the sub-motor should be appropriately provided with a back lash between the teeth to be meshed to prevent interference on steering. In the vicinity of the neutral position, a very small relative movement occurs between the teeth owing to the back lash so as to deteriorate the steer-holding characteristic in the vicinity of the neutral position.
Further in such a rear wheels steering apparatus, it is necessary to detect the stroke of the rod in the axial direction in order to control the steering. Since, however, the rod moves in the axial direction while rotating, it is not easy to directly measure the rod stroke. A method to indirectly detect by adding a gear mechanism to the reduction gear for an actuator is generally used. However, providing the reduction gear unit with an indirect rod stroke detecting mechanism accumulates back lashes, and causes an error between the actual stroke and the detected value, leading to deteriorated controllability having a large so-called "dead zone".
Also if a trouble occurs with another electric signal system, it takes time to shift to a fail-safe state by judging the situation, owing to the large error, and it is very unfit for a vehicle-mounting device.
In addition, a rear wheels steering apparatus for vehicles as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 57-44568 and 61-46763, equipped with the following has been known: an electrically-driven motor, a steering mechanism for steering the rear wheels by transmitting the driving force of the motor to the rear wheels side, and a controller for controlling the operation of the above electrically-driven motor in accordance with the handle steering angle, the vehicle speed, etc.
This type of rear wheels steering apparatus has a merit that the number of degrees of freedom is not limited in the control in steering the rear wheels, such as being able to steer the rear wheels even when the front wheels steering angle is zero as compared with a type for steering the rear wheels by transmitting the steering force of the front wheels to the rear wheels side through a rod, etc.
As a countermeasure against a failure of the electrically-driven motor in this type of rear wheels steering apparatus, the steering mechanism is generally provided with a clutch for cutting off the transmission of power from the electrically-driven motor to the rear wheels and a centering spring for energizing the rear wheels at the neutral position.
Since the preset load for the centering spring has been set to a considerably high value in this case, the rear wheels will be rapidly returned to the neutral position when the electrically-driven motor is out of order during traveling while turning. Therefore we have some fear for safety. Also, there is a problem that the electrically-driven motor will have larger size and a greater output to cope with the high preset load, and at the same time, the rear wheels will not be quickly steered.
To solve these problems, it is considered to provide two electrically-driven motors in parallel and to use one of these motors as a spare when the other is out of order. Since, however, a clutch for switching the power transmission system for the electrically-driven motor is required in this case, the apparatus will have larger size. Also since the driving motor for spare is rarely used, there is a problem in the control of the operation.